In vitro assessment of antimicrobial agents against toxoplasma gondii

We have modified a method of quantitating growth of Toxoplasma organisms by measuring incorporation of [³H]uracil into Toxop/asma-mfeacd, differentiated L₆E₉ rat myocytes and have found that low-dose pyrimethamine (0.1 µg/ml) and sulfadiazine (25 µg/ml) are synergistic. Pyrimethamine at higher concentrations (0.5 and 1.0 µg/ml) inhibits uptake to the same degree as the low-dose pyrimethamine-sulfadiazine combination. Spiramycin was effective only at high concentrations (200 µ g/ml) and with prolonged incubation of >72 h. Clindamycin and several of its analogues, methotrexate and difluoromethylor- nithine, were all ineffective and showed no additive effect with either pyrimethamine or sulfadiazine. Spirogermanium, an experimental antineoplastic and antiprotozoan agent, was effective only at concentrations close to those toxic to the system. 5-Fluorouracil was effective even at 0.1 µ g/ml. At 0.01 (µ g/ml it was synergistic with pyrimethamine (0.1 xg/ml), and the combination was as effective as high-dose pyrimethamine (1.0 µg/ml).